Large deposits of mineral trona in southwestern Wyoming near Green River Basin have been mechanically mined since the late 1940's and have been exploited by five separate mining operations over the intervening period. The nominal depth below surface of these mining operations ranges between approximately 800 feet to 2000 feet. All operations practiced some form of underground ore extraction using techniques adapted from the coal mining industry.
Trona ore is a mineral that contains about 90-95% sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O). The sodium sesquicarbonate found in trona ore dissolves in water to yield approximately 5 parts by weight sodium carbonate (Na2CO3) and 4 parts sodium bicarbonate (NaHCO3).
The crude trona is normally purified to remove or reduce impurities, primarily shale and other nonsoluble materials, before its valuable sodium content can be sold commercially as: soda ash (Na2CO3), sodium bicarbonate (NaHCO3), caustic soda (NaOH), sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O), a sodium phosphate (Na5P3O10) or other sodium-containing chemicals.
Soda ash is one of the largest volume alkali commodities made in the United States. Soda ash finds major use in the glass-making industry and for the production of baking soda, detergents and paper products.
To recover these valuable alkali products, the so-called ‘Monohydrate’ commercial process is frequently used to produce soda ash from trona. Crushed trona ore is calcined (i.e., heated) to convert sodium bicarbonate into sodium carbonate, drive off water of crystallization and form crude soda ash. The crude soda ash is then dissolved in water and the insoluble material is separated from the resulting solution. This clear solution of sodium carbonate is fed to an evaporative crystallizer where some of the water is evaporated and some of the sodium carbonate forms into sodium carbonate monohydrate crystals (Na2CO3.H2O). The monohydrate crystals are removed from the mother liquor and then dried to convert it to dense soda ash. The mother liquor is recycled back to the evaporator circuit for further processing into sodium carbonate monohydrate crystals.
The ore used in these processes can be dry mined trona obtained by sinking shafts of 800-2000 feet (or about 240-610 meters) or so and utilizing miners and machinery underground to dig out and convey the ore to the surface. Because of the mine depth and the need to have miners and machinery, the cost of mining the ore is a significant part of the cost of producing the final product. Additionally, trona beds, also known as trona seams, often contain thick bands of shale that must be removed as well during mechanical mining. The shale must then be transported along with the ore to the surface refinery, removed from the product stream, and transported back into the mine, or a surface waste pond. These insoluble contaminants not only cost a great deal of money to mine, remove, and handle, they provide very little value back to the operator.
One mining technique being developed to avoid the high cost of having miners and machinery underground is in situ solution mining. In its simplest form, solution mining is carried out by contacting a sodium-containing ore such as trona with a solvent such as water to dissolve the ore and form a liquor (also called ‘brine’) containing dissolved sodium values. The liquor is then recovered and used as feed material to process it into one or more sodium salts. The difficulty with trona solution mining is that trona is an incongruently dissolving double salt that has a relatively slow dissolving rate and requires high temperatures to achieve maximum solubility and to yield highly concentrated solutions which are required for high efficiency in present processing plants. Further, solution mining may also yield over time liquor solutions of varying strength, which must be accommodated by the processing plant.
Attempts of in situ solution mining of virgin trona in Wyoming were met with less than limited success, and were eventually abandoned in the early 1990's. Current in situ trona solution mining methods under development generally involve the directional drilling of borehole patterns horizontally through a virgin trona bed for some distance, the passage of a solvent (water) through the open borehole, and collecting the resultant trona liquor which is further processed for recovery of products. However, it is believed that these methods have an intrinsic limited productivity, since the maximum surface area available for dissolution is reached at the point where the trona seam around the borehole has been dissolved sufficiently to expose the insoluble roof and floor material. Once this point is reached, the only trona surfaces available for the solvent to react with are the walls (ribs) of the enlarged borehole. Therefore, meaningful volumes of solution can only be achieved by employing a very large number of very expensive boreholes.
Owing to the limited availability of ‘fresh’ trona surface area for the solvent to act upon, these methods can also be susceptible to a theorized phenomenon known as ‘bicarb blinding’ as well. Indeed, because sodium carbonate is more soluble than sodium bicarbonate, there is a tendency for the carbonate to go into solution more easily than the bicarbonate portion of the trona body. Thus, the exposed trona could leach to become less soluble bicarbonate and thereby ‘blind’ the unexposed trona.
In-situ solution mining methods are now currently employed for mining of remnant mechanically mined trona beds. A recent commercial trona mining technique that Applicants call ‘hybrid’ solution mining process takes advantage of the remnant voids left behind from mechanical mining to both deposit insoluble materials and other contaminants (collectively called tailings or tails) and to recover sodium value from the aqueous solutions used to carry the tails. Solvay Chemicals, Inc. (SCI), known then as Tenneco Minerals was the first to begin depositing tails, from the refining process back into the mechanically mined voids left behind during normal partial extract operation.
Hybrid solution mining processes are thus necessarily dependent upon the surface area and openings provided by mechanical mining to make them economically feasible and productive. These ‘hybrid’ mining processes cannot exist in their present form without the necessity of prior mechanical mining in a partial extraction mode. The associated ‘remnant trona’ left behind provides the volume of exposed trona necessary for meaningful production volumes while the openings left provide the volume needed for both solvent retention and liquor transport.
Even though solution mining of remnant mechanically mined trona is one of the preferred mining methods in terms of both safety and productivity, there are several problems to be addressed, not the least of which is the resource itself. Indeed, in any given mechanical mining operation there is a finite amount of trona that has been previously mechanically mined. When current trona target beds will be completely mechanically mined, the operators will have to start mining other less productive and more hazardous beds.
Also, since trona has relatively low solubility in water, in-situ hybrid solution mining systems make up for the low solubility of trona by introducing large volumes of water to large volumes of exposed trona for relatively long periods of time. Additionally or alternatively, the mining operator may use more aggressive solvents, such as caustic soda, to increase the solubility of trona, but it is generally believed that production cost is likely to become prohibitive at the scales necessary to provide meaningful production volumes.
Economically mechanically minable ore can be considered a valuable resource from another aspect as well. In current hybrid mining systems, the mechanically mined ore is essentially used to boost the total alkalinity (TA) of the ‘mine return water’ (MRW) solution. MRW typically contains from 12% to 20% TA. Calcined and leached mechanically mined ore is essentially used to raise the MRW alkalinity up to sufficiently high concentrations (+30%) as to be an economic evaporator feed for the monohydrate process. At ambient temperatures MRW becomes fully saturated at around 20% TA. If this liquor is introduced directly to an evaporator, a great deal of water must be boiled away to bring the concentration (and raise the temperature) up to +30% TA where soda ash crystal precipitation begins to take place. By employing both MRW and conventional calcining and leaching of mechanical ore, the MRW is increased in TA, thus making economic, mechanically mined ore a resource of even greater value.
Thus, a dilemma exists for trona mining operators. In order to remain competitive, the operator is encouraged to contain operations in the preferred target bed for as long as possible, but by doing so, the operator will eventually be forced to move a significant and ever growing portion of the operation into thinner beds of lower quality and to use more rigorous mining conditions while the preferred bed is depleting and finally becomes exhausted. Under this scenario, the competitive advantage enjoyed by today's trona operations in the global soda ash market will begin to dwindle over time and will likely end with the closure of the mines while available trona resources, yet to be mined, still remain in the ground. Current hybrid solution mining systems and mechanical mining systems (such as longwall mining) help to dramatically boost recovery of the mineral resource, but they only forestall the inevitable.
In addition to the need of large amount of solvent, limited productivity and probable limitation by ‘bicarb blinding’ for in-situ solution mining of trona beds, it was realized that in-situ solution mining of trona beds further suffers from decreased liquor quality. Indeed, the liquor may be contaminated with chlorides, sulfates and the like, which are difficult to remove when processing the liquor into sodium-containing chemicals. Not only does chloride contamination pose a problem for solution mining, it also causes severe issues in the downstream processes for refining the saturated solution (liquor).
This contamination can be explained as follows. While trona has relatively low solubility in water, chloride salts of some naturally occurring minerals in the roof shale above the trona, notably sodium chloride, are highly soluble. In fact, sodium chloride will displace the solubility of sodium carbonate and sodium bicarbonate to a significant degree. Due to chloride's high solubility, once chloride is in solution in the liquor, it is economically not feasible to separate it from the desirable solutes. The only way for the chloride salt(s) to leave the processing system is either through liquor purged to waste streams (carrying with it valuable mother liquor solution as well) or through the final product where chloride is a considerable contaminant for customers even at very small levels. In short, chloride contamination (also called ‘chloride poisoning’) of the pregnant sodium liquor during mining must be avoided.
The need to avoid chloride contamination poses a significant challenge to all in-situ trona solution mining processes, as the ‘chloride poisoning’ problem is derived from the environment of deposition of the trona beds. In the example of trona Bed 17 in Wyoming, the bed is bounded by a relatively impervious oil shale layer in the floor, and softer, more friable, ‘green shale’ layers in the roof and upper zones of the trona itself. It is these upper shales that pose the greatest potential for chloride poisoning of the solution mining liquor. Owing to the complicated process of deposition of the trona beds, the roof shales tend to contain significant amounts of chloride laden minerals, as well as other water soluble contaminants. If the roof shales are allowed to come in contact with the liquor in significant volumes (combined with fracturing and jointing) they are quite likely to ‘poison’ the liquor and render it unsuitable for refining. Therefore, it is desirable to carry out in-situ solution mining in such a way to avoid bringing significant volumes of these undesirable soluble minerals to come into contact with the solvent.
Moreover, the in-situ solution mining methods and systems can lead to wide spans of unsupported roof rock exposed to the solvent liquor. When these ‘open roof spans’ exceed a critical distance, ranging from only a few feet up to perhaps twenty feet, the roof will fail and fall into the solution-filled void along its entire length. Under these circumstances the roof shales literally soak in the solvent for nearly the full life of the borehole. Thus, chlorides, inorganics, and other soluble minerals will likely leach out of the shales and contaminate the liquor, rendering it useless.
This problem may be avoided, for the most part, in present hybrid solution mining of remnant pillars because the roof is not typically fractured and caved and allowed to soak in the solvent. The remnant pillars employed in this mining process holds the roof up out of the liquor as they are slowly dissolved away. The addition of insoluble tailings materials helps to stabilize a pillar and to avoid complete pillar failure as the pillar grows weaker and crumbles under overburden load during dissolution. Eventually, however, the void area around the pillar remnants is filled with insoluble material to the point where the surface of trona available to the solvent becomes insignificant and production declines until mining is eventually halted.
It is therefore desirable to carry out mining operations in such a way so as to conserve the more desirable trona resources suitable for mechanical mining, while at the same time extracting trona from less desirable beds without the negative impact of increased mining hazards and increased costs.
Ideally, trona should be extracted in such a way so as to minimize or even eliminate the need for mechanical mining in the trona beds, especially in these shallow trona beds which are currently less economically viable, and thus less desirable.